![viscosity of air viscosity of air](http://www-mdp.eng.cam.ac.uk/web/library/enginfo/aerothermal_dvd_only/aero/fprops/propsoffluids/mugraph.gif)
So, when this number is small, the viscous forces dominate and when it is large, the inertial forces dominate. This can also be thought of as the ratio of the inertial forces to the viscous forces. Where $u$ is the fluid velocity, $L$ is a representative length scale and $\nu$ is the kinematic viscosity of the fluid. What you need to compare when looking at bodies of different sizes and asking how the forces relate, is in general, the Reynolds Number as you included in your question. In other words, is it possible to scale up the insect flying "experience" to the human level, and get an idea of what the human equivalent of the viscosity involved is? I appreciate it may be impossible to answer this question without referring back to the flight dynamics of insects, in which case my apologies as there may be no current answer. Instead I wonder do we know, compared to the human experience with respect to the fluid viscosity difference between still air and water, what air "feels" like to move through for an insect, such as a mosquito? For this reason, this intermediate range is not well understood. The range of Reynolds number in insect flight is about 10 to $10^4$, which lies in between the two limits that are convenient for theories: inviscid steady flows around an airfoil and Stokes flow experienced by a swimming bacterium. This article implies that insect flight is still a subject of active investigation. I don't wish to ask a biology based question, or how any insect actually flies, which can be found at Insect Flight. On that sort of scale, I wonder is it possible to estimate how normal still air applies in terms of viscosity, to a mosquito or other similar sized insect, utilising standard fluid dynamics techniques? If I go for a swim though, I will immediately notice the viscosity of the water and the effort needed to move through it. If I go for a walk at, say 4 km/hour, unless there is a breeze blowing, I probably won't notice the air around me at all.